ocular SpeElra of Light and Colours • 327 
colour by an intermixture of theirs, as will be explained in 
another place. 
2. Place a lighted fpermaceti candle in the night about one 
foot from your eye, and look fteadily on the center of the 
flame, till your eye becomes much more fatigued than in S. IV* 
Exp. 3. ; and on clofing your eyes a reddifh fpe&rum will be- 
perceived, which will ceafe and return alternately. 
The action of vomiting in like manner ceafes* and is 
renewed by intervals, although the emetic drug is thrown up 
with the firft effort : fo after-pains continue fome time after 
parturition ; and the alternate pulfations of the heart of a viper 
are renewed for fome time after it is cleared from its blood, 
VI, OF REVERSE OCULAR SPECTRA. 
The retina after having been excited into a Elion by a Jiimulus 
fomewhat greater than the laft mentioned falls into oppojit# 
fp of mo die aElion . 
The actions of every part of animal bodies may be advan- 
tageoufly compared with each other. This ftridl analogy con- 
tributes much to the inveftigation of truth ; while thofe loofer 
analogies, which compare the phenomena of animal life with 
thofe of chemijftry or mechanics, only ferve to miflead our 
inquiries* 
When any of our larger mufcles have been in long or in 
violent action, and their antagonifls have been at the fame 
time extended, as foon as the action of the former ceafes, the 
limb is ftretched the contrary way for our eafe, and a pandicu- 
lation or yawning takes place. 
7 % 
